1. Field of the Invention
The present invention relates to an optical fiber capable of suppressing the stimulated Brillouin scattering.
2. Description of the Related Art
To realize a high-capacity optical communication, communication techniques such as the wavelength division multiplexing (WDM) and the time division multiplexing (TDM) are employed. In such communication techniques, as the intensity of an incident light input to an optical fiber used as a transmission line increases in the optical fiber, nonlinear optical effects increases in the optical fiber. The stimulated Brillouin scattering (SBS), one of the nonlinear optical effects, is a phenomenon that a part of the incident light is scattered backward and that the scattered light, i.e. the Brillouin-scattered light, stimulates further scattering. The SBS is caused by an interaction between the light and an acoustic wave in the optical fiber. The SBS is unfavorable because it disturbs light propagation in the optical fiber. The SBS occurs when the intensity of the incident light exceeds a predetermined threshold (hereinafter, “SBS threshold”), and therefore an optical fiber having a high SBS threshold is highly desired.
The SBS is also unfavorable for an optical fiber device used for optical signal control based on the nonlinear optical effects. Such an optical fiber device uses a highly nonlinear fiber having optical nonlinearity about five times higher than that of a typical optical fiber. The highly nonlinear fiber is realized by doping a large amount of GeO2 into a core of a silica-based optical fiber and decreasing an effective area of the core to increase confinement of the light.
The nonlinear optical effects employed in the highly nonlinear fiber include the self-phase modulation (SPM), the cross-phase modulation (XPM), and the four-wave mixing (FWM), which sometimes involve unwanted SBS. The SBS sometimes occurs with an intensity lower than the intensity with which the desired nonlinear optical effect occurs, and therefore a negative effect occurs in performing the optical signal control. Therefore, the highly nonlinear fiber also requires a high SBS threshold.
To increase the SBS threshold, there are conventionally suggested methods of varying optical properties, such as the wavelength dispersion and the transmission loss, in the longitudinal direction of the optical fiber by changing the core diameter and/or an amount of a dopant doped into the core in the longitudinal direction (see, for example, Japanese Patent No. 2584151, Japanese Patent No. 2753426, and Japanese Patent No. 3580640). In the above methods, because a shift amount of the Brillouin-scattered light (Brillouin shift amount) with respect to the incident light on the optical frequency spectra varies in the longitudinal direction, such that the SBS threshold increases and the SBS is hardly generated.
However, although the SBS can be suppressed with the increased SBS threshold in such an optical fiber with the optical properties varied in the longitudinal direction, there is a drawback that the optical properties are not constant in the longitudinal direction. Particularly, a pulse shape of an optical pulse can be distorted when the optical pulse input to the optical fiber passes a portion of the fiber where the optical properties are locally changed. Furthermore, though the optical properties need to be more precisely adjusted as the signal transmission rate increases, if the optical properties of the optical fiber vary in the longitudinal direction, the optical signal can be degraded like the optical pulse. Moreover, because efficiency of the nonlinear optical effects in the highly nonlinear fiber is affected by the local wavelength-dispersion properties, the efficiency can be decreased if the optical properties of the optical fiber vary in the longitudinal direction.